In wireless communication systems, mobile handsets communicate with other mobile handsets through base stations connected to the PSTN (public switched telephone network). Typically, in FDMA systems the base stations determine the frequencies at which the handsets are to communicate and send signals to the handsets to adjust the transmission power of the handsets.
The signals that are transmitted by the handsets are typically amplified prior to transmission to the base station. The amplification of the signal within the handset is generally performed by a radio frequency (RF) power amplifier 10, a representative embodiment of which is depicted in FIG. 1 (PRIOR ART). The RF power amplifier 10 includes a DC power terminal 12 and ground terminal 14. A DC power source 16 is typically connected between the power terminal 12 and the ground terminal 14, producing a supply voltage, V.sub.S, at the power terminal 12 and a supply current, I.sub.S, into the power terminal 12. Thus, the RF power amplifier is supplied with a DC power, P.sub.DC, equal to V.sub.S *I.sub.S. An RF input signal, RF.sub.in, generated by the transmitting handset, is fed into the RF power amplifier 10 via an RF input terminal 18. The RF power amplifier 10 amplifies the RF input signal, RF.sub.in, to produce an RF output signal, RF.sub.out, at an RF output terminal 20. The RF output signal, RF.sub.out, after passing through signal processing circuits, is typically sent to the antenna for transmission. An RF input signal, RF.sub.in, has an average input signal power, P.sub.in, and an RF output signal, RF.sub.out, has an average output signal power, P.sub.out.
When transmitting a signal with a non-constant envelope from a handset it is desirable to operate the power amplifier 10 in a linear mode to minimize signal distortion and bandwidth required to transmit the signal. The linearity of the power amplifier, which is measured by the uniformity of the transfer characteristic (P.sub.out /P.sub.in), varies with I.sub.S, V.sub.S, and RF.sub.out. Referring to FIG. 2 (PRIOR ART), the curves C1, C2, and C3 represent compression characteristics of an RF power amplifier 10 of FIG. 1, given three exemplary amplifier DC power, P.sub.DC, levels. The line L represents linear operation of the amplifier 10. As curves C1, C2, and C3 illustrate, the linearity of the power amplifier depends on P.sub.DC. That is, as P.sub.DC increases, the range of P.sub.in values for which the amplifier remains linear increases. In general, the output power, P.sub.out, for which a power amplifier compresses increases with the DC power supplied to the power amplifier.
Although supplying a relatively high DC power to the RF power amplifier 10 will generally maintain linear operation of the RF power amplifier 10, such an arrangement becomes less advantageous in a system with varying transmission power requirements. A wireless communications system restricts the transmission power of the handset to minimize the signal from propagating to an excessively far point, so that the same frequency may be used at a far point, i.e., in other cells in order to permit servicing of as many subscribers as possible within the finite frequency resources allocated to the system. At the same time, the transmission power must be high enough to maintain the integrity of the transmitted signal over the distance that it travels to a base station. The magnitude of the handset transmission power required to maintain proper communication with a base station is dictated in part by the distance and the electrical communication environment between the handset and the base station. That is, if the handset is located far from a base station, the level of the RF output signal power, P.sub.out, will be relatively high. If the handset is located close to the base station, the level of the RF output signal power, P.sub.out, will be relatively low.
In a situation requiring a relatively low handset transmission power, an RF power amplifier that is supplied with a high DC power is inefficient. Referring to FIG. 1, the power the power amplifier 10 dissipates as heat is equal to the difference between the power supplied to the RF amplifier 10, P.sub.DC and P.sub.in, and the RF output signal power, P.sub.out, as characterized by the equation, P.sub.HEAT =P.sub.DC +P.sub.in -P.sub.out. Thus, given a constant DC supply power, P.sub.DC, the lower the RF output signal power, P.sub.out, is, the more power the amplifier wastes as heat. The wasted power in the power amplifier 10 can be quantified in the power efficiency equation, P.sub.eff =P.sub.out /(P.sub.DC +P.sub.in). Thus, the more DC power that is supplied to an RF power amplifier, the less efficient that RF power amplifier becomes for a constant P.sub.in and P.sub.out.
Therefore, it can be understood that an RF power amplifier that is supplied with a relatively high constant DC power generally operates linearly over a full range of RF output signal power levels, but is power inefficient, thus leading to significantly increased battery and heat sinking requirements, heavier battery weight, and shorter battery life. On the other hand, a power amplifier that is supplied with a relatively low constant DC power is power efficient, but generally operates only linearly over a low range of RF output signal power levels, thus resulting in a distorted transmission signal with a larger bandwidth.
There thus remains a need to operate a power amplifier more efficiently and linearly over a full range of given RF signal output power levels.